Typically in some cameras the film transport mechanism is motorized. In operation, a leading end portion of the filmstrip protruding from a lighttight film cartridge loaded in the camera is attached automatically to a film take-up spool. The take-up spool is rotated after each exposure to advance successive frames of the filmstrip from the cartridge, across the focal plane of a taking lens, and onto the take-up spool. Each time the take-up spool is rotated, an unexposed frame of the filmstrip is positioned in the focal plane of the taking lens and an exposed frame is wound onto the take-up spool. When all of the available frames have been exposed, a trailing end portion of the filmstrip remains attached to a supply spool inside the cartridge. This end-of-film condition produces a sudden increase in the film tension and in the motor current as the take-up spool attempts to withdraw the remainder of the filmstrip from the cartridge. A tension sensing device responds to the increase in the film tension, or a current detecting circuit responds to the increase in the motor current, by reversing the motor drive to rotate the supply spool inside the cartridge. The rotated spool draws the exposed frames off the take-up spool and rewinds them into the cartridge.
Alternatively, in place of a tension sensing device or a current detecting circuit, a perforation sensor may provided for detecting a turn-around perforation in the trailing end portion of the filmstrip to initiate return of the filmstrip from the take-up spool to the cartridge.